Alpine Lake Chemistry: A long-term study of water quality in the alpine lakes of Ecoregion 2

Maeve McCormick, Chuck Rhoades, and Tim Fegel. Spring, 2021.


Getting oriented - the where, what, and why?

What is an ecoregion, and where is Ecoregion 2?

The United States Forest Service (USFS) defines ecoregions as "large areas of similar climate where ecosystems recur in predictable patterns." They are official administrative boundaries that help organize scientific studies and surveys in standard areas. Ecoregion 2 encompasses most of the intermountain western US. In other words, the drought-prone, water-limited region around the Rocky Mountains, characterized by high-elevation forests, meadows, riparian areas, and prairie ecosystems.

ecoregion2_USFS_map.png Figure 1 image adapted from the USFS RMRS Ecoregions page.

The USFS has been performing a long-term survey of alpine (high-elevation) lake water quality in Ecoregion 2 since the early 90’s. This study includes lakes from Idaho, Montana, Utah, Wyoming, and Colorado. Water samples are collected from these lakes and shipped to the USFS Rocky Mountain Research Station (RMRS) in Fort Collins, CO. The RMRS lab measures ion concentrations, pH, acid neutralization capacity (ANC), and specific conductivity (SC) in each sample it recieves.

Study Area

I filtered the original database of lakes to focus on those with the longest uninterrupted period of data collection. As a result, my analyses use lakes primarliy located in in Wyoming and Colorado. That said, my workflow should be easily adaptable to include more lakes within this region, and even other ecoregions.

lake_locations_CO_WY.png

Figure 2: A study area map of the sub-selected lakes. Basemap courtesy of OpenStreetMap contributors, accessed with the contextily python package.

Why alpine lakes?

You may be wondering why alpine lake chemistry is so important. These lakes are at high elevations! Some are very small, and tucked away in basins not easily accessible. It's true these lakes are remote, but they are vital water sources. Alpine lakes feed the streams that become tributaries to major rivers, which serve as cruicial water resources for the communities of the intermountain west. Alpine lakes are the ultimate headwaters for western watersheds, and changes in lake chemistry are known to propegate downslope and downstream. These lakes serve as sources and buffers for mountain streams - they store ions and neutralize acids, which helps regulate pH and nutrient abundance (nitrates and phosphates are important in nutrient-limited systems, but an over-abundance can result in oxygen depletion and other cascading environmental effects).$^{[1]}$

The communities that depend on these vulnerable water sources are growing, and with that growth comes increases in anthropogenic emissions - we are releasing more nitrates, phosphates, and sulfates to the atmosphere via industry and agriculture.$^{[1]}$ These compounds end up in lakes and soils via atmospheric deposition, and ultimately end up in other parts of watershed ecosystems via the hydrologic cycle. We need to understand how water chemistry is changing in alpine lakes so that we can inform water resource management decisions, and more effectively prioritize long-term water quality and sustainability. atmospheric_deposition_USFS_fig.png

Figure 3: A simplified diagram describing atmospheric cycling of nutrients, and atmospheric deposition. Atmospheric deposition is part of a natural cycle of nutrient exchange, but anthropogenic emissions introduce an impbalance in the cycle, resulting in more of these compounds concentrating in lakes via acid precipitation. Image from the USFS Air Resource Management Program.

Answering Questions with Geospatial Analyses


In order to investigate changes in alpine lake chemistry, I developed the following workflow using Python and publicly-available data stored in open source formats.

Does geology affect baseline levels of ions in lakes?

  1. Overlay the lake locations on a geologic map (I am still working to acquire digitized geologic maps, so I used a placeholder terrain map for the figure above). Group the lakes according to broad geologic categories (such as geologic units, or even more general rock type categories).

How closely correlated are ion concentrations to water quality parameters?

  1. Generate comparison plots for water quality parameters (such as pH, ANC, and Specific Conductivity) and ion/nutrient concentrations. For an example, see Figure 4 below.

To what extent is alpine lake chemistry changing over time?

  1. Generate timeseries plots for each ion concentration and water quality parameter at each available lake.
  2. Create a classification scheme based on the geologic units associated with each lake, and group the lakes according to those classifications. (Future process.)
  3. Repeat the process for generating timeseries plots with the lakes grouped by geology, and assess any relationships that emerge from the data. (Future process.)
  4. Based on the conclusions we are able to draw from these results, we will consider how to incorporate data about atmospheric deposition of nitrates and phosphates (available from the National Atmospheric Deposition Program (NADP)) in future analyses.

Preliminary Results


Ca_SC_complot_2-2.png

Figure 4: A comparison plot of Calcium (Ca) Concentration and Specific Conductivity (SC) for all of the available lakes. Note there are several outliers on the lefthand side of the plot, likely due to blank samples still being included. There is still a positive correlation between Ca Concentration and SC, as expected.

timeseries_multiplot1.png

Figure 5: Sevreal draft timeseries plots for different lake chemistry parameters. Note, all have a gap in the data from 2012-2016, and are not yet grouped by geology. Once the lakes are classified by geology, these plots will be cleaned.

Conclusions?


This work is still in preliminary stages. More time, data cleaning, and analyses are necessary before we can make any interpretations from these data. So, here are the planned...

Next Steps!

Works Cited and additional resources

  1. Musselman, R.C., Slauson, W.L. Water chemistry of high elevation Colorado wilderness lakes. Biogeochemistry 71, 387–414 (2004). https://doi.org/10.1007/s10533-004-0369-6

Data resources:

This work is part of an ongoing project, and the lake chemistry data used are not yet published. As the data were collected by the USFS, they will be published with associated metadata according to federal data management guidelines, and will be freely available to the public. If you wish to work with the preliminary, unpublished data, please contact Maeve McCormick (mmccorm2@alumni.stanford.edu).

Additional sources will be appended as map resources are selected. For those curious, the USGS hosts a National Database of geologic maps, found here: https://ngmdb.usgs.gov/ngmdb/ngmdb_home.html.

Code resources:

To see the code used to generate the above maps and figures, please contact Maeve McCormick (mmccorm2@alumni.stanford.edu).